Still receiving invoice after subscription transfer

I have recently moved out of my old apartment and I want to transfer my internet subscription (Horizon Mini Trio) to the new tenant. To achieve this, I submitted the service transfer form to UPC two months ago, stating that the owner-chance must be effective by August 1. The form was received and acknowledged by UPC. However, I still received the invoice for this month (August), when in fact, this invoice should have been addressed to the new tenant instead. I have been trying to contact UPC for the past two weeks via emails, phone calls, twitter, to no avail (i.e., no responses at all). This is driving me crazy. I would appreciate any help from the forum.